Thin-wall polytetrafluoroethylene (hereinafter PTFE) tubes are useful for a variety of applications where the thinness of the tube wall and the lubricity of the PTFE are advantageous. The chemically inert character of the PTFE is also advantageous, particularly when the biocompatibility of the tubing is a concern.
Conventional thin-wall PTFE tubes suffer from poor flexibility and have relatively poor mechanical strength characteristics, specifically tensile strength and hoop strength. Their resistance to creep when subjected to mechanical loads for extended periods of time is also poor. These conventional tubes also possess significant memory in that if subjected to, for example, a crease or kink, the tube will remember the crease or kink and will want to assume the same crease or kink subsequently if exposed to the slightest causative force.
U.S. Pat. No. 4,925,710 to Buck et al., describes a thin-wall sintered fluoropolymer tube having a wall thickness of less than about 0.051 mm. The tube is made by extruding a non-porous fluoropolymer tube over a fluoropolymer core containing a filler and subsequently removing the core. U.S. Pat. No. 4,791,966 to Eilentropp describes a PTFE tube made by helically wrapping a PTFE tape around a mandrel, sintering the wrapped tape to fuse the overlapping tape edges and finally removing the mandrel from the resulting tube. The tape is relatively thin, from 30 to 300 micrometers in thickness, and is of trapezoidal cross section wherein the tape edges are thinner than the center of the tape width, so that the overlapping edges of the helically-wrapped tape result in a tube of relatively uniform wall thickness. The tubes of Buck et al., and Eilentropp both suffer from the aforementioned disadvantages of poor mechanical strength characteristics, poor flexibility, and excessive memory.
U.S. Pat. Nos. 3,953,566; 3,962,153 and 4,197,390 to Gore describe making tubes and sheet films of porous expanded PTFE wherein the PTFE has a microstructure of nodes interconnected by fibrils. The tubes are made by extruding a paste of a mixture of PTFE fine powder and a liquid lubricant, removing the lubricant from the resulting tubular extrudate, expanding the extrudate by stretching at a suitable rate at a temperature less than the crystalline melt point of the PTFE, and preferably sintering the expanded PTFE tube while the tube is longitudinally restrained. These patents do not teach the construction of thin-wall tubes. Tubes made according to these patents are commercially available as vascular grafts (W. L. Gore & Associates, Inc., Flagstaff Ariz.) and are provided with an exterior helical wrapping of porous expanded PTFE film which increases the hoop strength of the tube.